# Physics and a Bicycle

Discussion in 'The Bike Cafe' started by Michael Pisarri, Apr 7, 2005.

1. ### Michael Pisarri New Member

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Does anybody know why a bicycle will stay up just fine when your moving but when you are stopped its almost impossible?

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2. ### Jaguar27 New Member

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Yes, it's called "gyroscopic precession" if you google those two words there should be a ton of info on it...(saves me some typing)...in fact, a Bicycle is one of the best examples of this...not to mention the most fun

3. ### greg-fl450 New Member

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While gyroscopic precession is a property of gyroscopes, the property that allows the bike to remain upright while riding is called "gyroscopic rigidity" (or something like that). That is the tendency of the spinning body (bike wheel) to to remain rigid in space (upright). Precession is another neat property of all spinning bodies, but isn't what keeps the bike upright.

4. ### TrevorN New Member

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a fun way to mess around with this is to sit in an office chair that can spin and take the front wheel off your bike. sit in the chair and spin the wheel as fast as you can and then hold it like your the fork. then tilt it slightly and you can get your chair to move and it makes your arms feel funny. haha

5. ### Count Dz New Member

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I know someone with just a little too much time on their hands during office hours . Back to work young man.

6. ### ganderctr New Member

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Actually, gyroscopic forces aren't present in significant quanities until you get over about 15 mph on a bicycle. Countersteer is what keeps a bike upright at slower speeds. Riding no-hands is harder below about 15 mph because below that, the gyro forces haven't built up enough to keep the front wheel pointing basically straight, even though steering is fine below that speed. Ice skaters can balance on one skate because they steer to balance; no gyroscopic forces are needed to keep them upright. Bicycles behave similarly at low speeds. Ever tried to ride at below 2 mph? You'll really feel it down there.

7. ### Walrus New Member

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I have a pen.

8. ### ganderctr New Member

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That'll work. Balance it in your hand.

9. ### BassDave New Member

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You know, that is interesting, about riding at or below 2 mph. It can be done, but your body is adjusting a lot in order to keep you upright. It isn't just physics in play, but the processing/reaction speed of the brain that amazes me. Try to create the same simple task with a computer controlled 'robot' [or whatever], and you'll spend millions of dollars and endure countless failures to reproduce it.

10. ### Hypnospin New Member

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not impossible, really. when i started toe clips 'n straps were the rule. a guy i rode with introduced me to track stands. i asked, "how long can you do that for?"
answer: "long as i want" and this guy was not boasting.
i still do the trackstand even with clipless, not often though as i seldom stop.

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A great example of "gyroscopic procession", also called "phase lag" sometimes, is the control input from the cyclic to the rotor blades in a helicopter. It's a pretty weird phenomenon. Any input made in the gyroscope (in this case the rotor disc) doesn't manifest itself until 90 degrees later. And I would argue that this is equally as fun as cycling! Of course the control rods of any modern helicopter compensate for this so the pilot moves the cyclic in the direction he really wants the disc to move, but poor Igor Sikorsky flew his first prototypes with no such compensation...if he wanted to tilt the disc fore, he had to move the cyclic right, etc. Yikes!

12. ### JSWin Member

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Is this a trick question? Because its designed that way and there are no training wheels.

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